hikey: Build boot_fat.uefi.img only if TARGET_BOOTIMAGE_USE_FAT=true am: e8886ddf2e
am: 9888d33ae1

* commit '9888d33ae1f2cc34200cd1f5d799d2dd009e7135':
  hikey: Build boot_fat.uefi.img only if TARGET_BOOTIMAGE_USE_FAT=true

Change-Id: I19d28d537febed4e44335300e869d3c265b601f0